In general, digital transmission of image signals requires high compression. For example transmission methods of image signals include H.261 for teleconferencing, MPEG I for multimedia, MPEG II for multipurpose use such as digital TV, and a compression method for HDTV, all of which employ, for high compression, a method for compressing through compensation of motion so as to eliminate duplication existing on time base.
In order to raise compression ratio of H.261 and MPEG I, dissemination is carried to lower the frame ratio for coding, other than the method for compression through compensation of motion as explained above. That is, compression is carried out with a lowered frame ratio of 25 Hz or 30 Hz.
However, even though transmission is carried out compressed with a lowered frame ratio to 25 Hz to 30 Hz, the compressed signals should be converted into signals with a frame ratio of 50 Hz or 60 Hz for final display on a monitor.
To accommodate film modes with a frame ratio of 24 Hz or 30 Hz, film mode signals compressed to a frame ratio 24 Hz or 30 Hz for transmission should be converted into signals with a frame ratio of 60 Hz to facilitate displaying on a monitor at the final display stage.